Bipolar transistors (BJTs) are commonly used in semiconductor devices especially for high speed operation and large drive current applications. A single polysilicon BJT 10 is shown in FIGS. 1a-b. The area for the BJT 10 is isolated by field oxides 12. The collector 14 is a lightly doped epitaxial layer of one conductivity type and the base region is formed by doped regions 16 and 18 of the opposite conductivity type. Doped region 16 is called the intrinsic base region and region 18 is the extrinsic base region. The extrinsic base region 18 provides an area for connecting to the base region. The emitter region 22 is a doped region of the same conductivity type as the collector and is located within the intrinsic base region 16. The emitter electrode 24 is accomplished with a doped polysilicon layer. Emitter dielectric layer 26 isolates the emitter electrode 24 from the base regions 16 and 18. Both the extrinsic base regions 18 and the emitter electrode 24 are silicided to reduce resistivity. Thus, there is a silicide contact 30 over the extrinsic base regions 18. Single polysilicon BJTs can be fabricated with a less complex process than double polysilicon BJTs. However, they typically have the disadvantage of higher base resistance and increased extrinsic capacitances over double polysilicon BJTs.
The BJT of FIG. 1 is typically formed by forming a screen oxide over the silicon active area and implanting a base region. The screen oxide is then thickened to form emitter dielectric layer 26. Next, an opening is etched in the emitter dielectric layer 26. A thicker emitter dielectric is desired to reduce polysilicon emitter capacitance. However, because the base is implanted before the emitter pattern, etching the emitter dielectric can result in an overetch that varies device parameters if a thick emitter dielectric is used. Next, the emitter electrode 24 and emitter region 22 are formed. The emitter electrode 24 is patterned with over two alignment tolerances. Alignment tolerances need to account for the alignment of the emitter electrode to the emitter, the lateral diffusion of the extrinsic base implant and the alignment tolerance for the extrinsic base implant. Accordingly, the emitter electrode 24 can extend greater than 0.5 .mu.m over the emitter dielectric 26 layer as shown in FIG. 1. The extrinsic base regions 18 are then implanted, diffused, and subsequently silicided. However, the base link-up distance 32 dominates the base resistance. The base link-up distance 32 is determined by the size of the emitter electrode 24. As discussed above, the size of the emitter electrode 24 is determined by the amount of alignment tolerances necessary. This requires a double sided base contact 30 for low resistance, limits the minimum device area, and requires deeper extrinsic base region 18 to prevent punch-through of the junction corner when siliciding the base contacts. It also reduces Fmax (the unity power gain frequency), limits high current operation and increases amplifier noise in low impedance amplifiers. Accordingly, there is a need for a BJT that overcomes the above problems.